Q&A - P5 Math

[kkme]

Hi, need help for these two qns.


1) Tom’s present age is thrice Sally’s age some years ago. Conversely, Sally’s present age is Tom’s age the same number of years ago. Given that the sum of their present ages is 30 years, how old is Tom now?

2) When Kok Hwa’s age was twice Bala’s age, Mike’s age was 30. When Mike’s age was twice Kok Hwa’s age, Bala was 21. Given that Mike was the oldest among the three persons, find Bala’s age when Mike was 62 years old.

Thanks in advance and Happy CNY!

[tianzhu]

kkme:

1) Tom's present age is thrice Sally's age some years ago. Conversely, Sally's present age is Tom's age the same number of years ago. Given that the sum of their present ages is 30 years, how old is Tom now?

Hi

For Q1, you may use MD or Units and Parts.

Due to time constraints, I’ll skip the MD this time.

At present
Tom ----- 3 units
Sally ----- 1 part

Some years ago
Tom ----- 1 part
Sally ----- 1 unit

The difference in age between two people remains the same.

3 units – 1 part ----- 1 part – 1 unit

4 units ----- 2 parts
2 units ----- 1 part

The sum of their present ages is 30 years.

3 units + 2 units ----- 30
5 units ----- 30
1 unit ----- 6

Tom ----- 3 units ----- 18

Best wishes

[tianzhu]

kkme:

2) When Kok Hwa's age was twice Bala's age, Mike's age was 30. When Mike's age was twice Kok Hwa's age, Bala was 21. Given that Mike was the oldest among the three persons, find Bala's age when Mike was 62 years old.

Hi

For Q2, due to time constraints, I'll provide some pointers instead of drawing MD.

Case 1
Bala ----- 1 unit
KH ----- 2 units
Mike ------ 30

Difference in age between KH and Bala ----- 1 unit

Difference in age between Mike and KH ----- 30 – 2 units

Case 2
Bala ----- 21
KH ----- 21 + 1 unit
Mike ------ (21 + 1 unit) + (21 + 1 unit)
Difference in age between Mike and KH ----- 21 + 1 unit

Difference in age between two people remains the same.

30 – 2 units ----- 21 + 1 unit

3 units ----- 9
1 unit ----- 3

When Mike’s age was 30, Bala was 3 years old.

62 – 30 ------ 32

Therefore , Bala ----- 3 + 32 ------ 35 years old.

Best wishes

[MathIzzzFun]

kkme:

Hi, need help for these two qns.


1) Tom's present age is thrice Sally's age some years ago. Conversely, Sally's present age is Tom's age the same number of years ago. Given that the sum of their present ages is 30 years, how old is Tom now?

2) When Kok Hwa's age was twice Bala's age, Mike's age was 30. When Mike's age was twice Kok Hwa's age, Bala was 21. Given that Mike was the oldest among the three persons, find Bala's age when Mike was 62 years old.

Thanks in advance and Happy CNY!

Hi

MD for Q2 which is same as this question that was discussed some time ago - http://www.flickr.com/photos/62167097@N02/5679985631/in/photostream

cheers.

[mathnoobs]

I’ve a question taken from the book by Ammiel Wan, Challenging Maths Problems Made Easy. This is from the chapter on Gap and Difference, Q7.


If I buy 3 popcorn combos and 4 nacho combos, I will have $4.50 left. If I buy 4 popcorn combos and 3 nacho combos, I will have $3.50 left. Given that a nacho combo costs $6, find the amount of money I have.

Difference between popcorn combos = 1 popcorn combo
Difference between nacho combos = 1 nacho combo
Difference between costs = $1

Here is the part I don’t understand, the solution says:
1 popcorn combo - 1 nacho combo = $1 (popcorn costs more than nacho)

why not,
1 nacho combo - 1 popcorn combo = $1 ? (nacho costs more than popcorn)

how do I know which costs more, popcorn combo or nacho combo ?

[MathIzzzFun]

hi


drawing a model will help you see that popcorn cost more than the nacho ..

http://i41.tinypic.com/14bjncz.jpg\">

cheers.

[happyheart]

Urgent! Mental block.

How to explain this question using algebra substitution method? e.g 5p=4e

Q: Two identical pens cost as much as three similar erasers. Sally bought 5 such pens and 4 such erasers at $9.20.
What is the cost of 1 pen?

Thanks!

[tianzhu]

happyheart:

Urgent! Mental block.

How to explain this question using algebra substitution method? e.g 5p=4e

Q: Two identical pens cost as much as three similar erasers. Sally bought 5 such pens and 4 such erasers at $9.20.
What is the cost of 1 pen?

Thanks!

Hi

You may use “Number*Value”

Use a common multiple of 6 (2 and 3)

2 pens ----- 6 units
1 pen ------ 3 units

3 erasers ------ 6 units
1 eraser ------- 2 units

Sally bought 5 such pens and 4 such erasers at $9.20.

(5*3) + (4*2) ------ 920

23 units ------ 920
1 unit ------ 40

1 pen ------ 3 units ------- 120 cents or $1.20

Best wishes

[happyheart]

Appreciate your prompt reply, Tianzhu.


Thank you!

[mathnoobs]

Hi Mathizzfun

Thank you for taking the trouble to draw out a diagram.
I'm afraid I still don't get it.
I've drawn a model here that says Nacho combos costs more than Popcorn combos.
http://i42.tinypic.com/2laa1hj.png\">

can you tell me what is wrong with the diagram ?

Is there a way to tell which is bigger without resorting to diagram ? My child's motor skill is not very good, and has difficulty in getting the relevant boxes to be of the same size.